Apparatus for fusing an electroscopic powder particle image



Y L. LEVINSON 3,437,072

APPARATUS FOR FUSING AN mscwaoscorzc POWDER PARTICLE IMAGE April 8, 1969 Filed April 15, 1965 FURNACE WATEYZ SUPPLY m/ v/vro/2 L 50 LE v/lvs 5 v f United States Patent US. Cl. 118-9 5 Claims ABSTRACT OF THE DISCLOSURE An apparatus for fixing a powder image which is deposited on a surface of a thin plastic material comprising means for applying a predetermined duration heat pulse and means for applying a cooling spray to the opposite surface of the plastic material.

This invention relates to a system for fixing powder ink of the type employed in electrostatic printing to a surface on which a powder image has been deposited and more particularly to improvements therein.

In an electrostatic printing system of the type described in Patent No. 3,081,698 by Clyde Childress et al., electroscopic powder particles are pushed through the apertures of a screen into an electric field which is established between the screen and an image receiving member to be directed onto the image receiving member. The screen has the apertures therein disposed in the pattern of a desired image. The powder particles, which are dry, are held to the image receiving surface by electrostatic forces of adhesion. However, the powder deposit is not permanent and can be dislodged or smeared by direct contact. In order to make the image permanent, the powder is usually heated sufficiently to cause the powder particles to flow together and wet the image receiving surface. To produce a reliable permanent bond between the powder and the image receiving surface, enough energy must be supplied to insure that the surface on which the dry powder image has been deposited comes up to a temperature of at least that of the flow point of powder. If this requirement is not met, the powder immediately adjacent to the image receiving surface cannot fuse or wet the image receiving surface and the resulting fix is unreliable.

The dry ink powder used in a typical electrostatic printing system has a flow point of at least 180 F. and usually considerably higher. This means, that in order to obtain proper fixing, the image receiving surface temperature must be at least 180 F., and with certain inks might run considerably higher. It will be appreciated that with many image receiving materials, such as a thin plastic, for example, special pains must be taken to insure that the elevated temperature of one surface of the plastic does not cause sufficient loss of structural integrity to allow the plastic object to deform or warp. In very thin sections, plastic material has a low heat capacity and, when subjected to a high heat flux, will very quickly come up to the softening temperature, not only on the surface but also through the whole cross-section of the material.

An object of this invention is the provision of a unique arrangement for uniformly fusing electroscopic printing powder to either flat or embossed, and to either uniform thin or greatly irregularly thin, plastic film surfaces without deforming, warping or degrading the plastic film or surface.

Yet another object of this invention is the provision of a novel arrangement for heating electroscopic powder particles which have been deposited upon a surface of a 3,437,072 Patented Apr. 8, 1969 thin plastic material in a manner to cause a fixing of the powder to the surface to obtain a glossy or matte powder finish as desired.

These and other objects of the invention are achieved by subjecting the powder image, which is deposited upon one surface of a thin plastic material, to a predetermined pulse of heat having a predetermined duration while cooling the opposite surface of said thin plastic material.

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention itself, both as to its organization and method of operation as well as additional objects and advantages thereof, will best be understood from the following description when read in connection with the accompanying drawing, which is an exploded view of an embodiment of the invention.

Referring now to the drawing, it is desired to fix an electroscopic powder particle image 10, which has been deposited on the surface of a plastic container 12, using a suitable technique, such as is described in the aforesaid patent to Childress et al. The container 12 may be, for example, an embossed plastic cigarette container with a typical gauge thickness varying on an average container from one and one-half mils, or thousandths of an inch, to 20 mils. A furnace 14 having a radiating area 16 is positioned spaced from and opposite the location at which the plastic container 12 is to have its image exposed to the heat from the furnace for fixing. In order to insure that each point on the image area of the container receives an exposure through essentially of solid angle, the furnace and the container must be as close together as possible.

Interposed between the furnace and the container there is positioned a rotatably driven shutter 18, having an arcuate opening 20 therein for exposing the powder image it to the radiating area of the furnace over a predetermined interval. A spray of a mixture of water and air is blown into the container parallel and adjacent to the surface directly back of the power image, making intimate moving contact with this surface. Any suitable arrangement for providing the air-water spray may be provided. This is exemplified by water from a supply 22 being drawn up through a nozzle 24 by means of air being blown by a pump 26 into said nozzle.

By way of example, and not be construed as a limitation on the invention, an embodiment of the invention was built and operated successfully for the fixing of an electroscopic powder image of about three square inch area printed on an embossed plastic cigarette container. The radiating area 16 of the furnace was made to be approximately 20 square inches. The shutter opening 20 was made four inches wide and had six inches of arc. The components were brought together with a clearance of approximately Vis t0 inch between each part. The radiating area of the furnace, as well as the shutter opening, was appreciably larger than the powder image to be fixed to insure the exposure of each image point to nearly a full hemispherical radiant surface. The coolant spray was directed into the open bottom end of the container so as to thoroughly but gently wet and cool the inside of the container wherever the corresponding outer surface was heated by the furnace. The spray flows in a pattern directly adjacent and parallel to the plastic surface leaving the container center and the container opening center for spray exit.

The shutter shown, when rotated at a speed to give an exposure time of approximately of a second, provided a heat pulse with the proper characteristics to fix the powder image. By decreasing or increasing the are opening and, accordingly by thereby increasing or decreasing the rotating speed of the shutter to keep the same exa posure time, the heat pulse rise time may be increased or reduced at will. The heat input pulse shape thus obtained can affect the physical characteristics of the fixed image. For example, it has been found that the gloss of the powder image can be varied by changing the shape of the heat pulse. In general, a slow rise time gives more gloss than does a fast rise time for the same exposure. A fast rise time will produce a flat or matte finish, particularly at the higher furnace temperatures. An excessively rapid rise time at the highest temperatures used will result in a grainy or damaged fix. This is notwithstanding the fact that exposure time is unchanged. Different powders will, of course, react differently to any changes in pulse shape or size depending on their composition and interaction with the plastic substrate.

Furnaces which may be used are types used in high temperature laboratory work. The typical unit shown in the drawing consists of an insulating refractory body coated inside with a dark material (i.e., silicon carbide grain) supported and cushioned within a steel shell by ceramic fiber. Heat is derived from a close, deep array of silicon carbide heating elements. This construction enables the short, high-density bursts of energy required, while still permitting operation at relatively low temperatures so that all ink colors, light and dark, may be equally fixed. Typical incident heat flux levels are from 15 to 250 watts per square inch, transmitted from radiating surfaces at from 1800 F. to 2450 F. Average exposure time on most substrate materials is to (9, second, except in the case of metals and other good thermal conductors.

The effect of this sudden burst of intense energy is to uniformly and rapidly raise the temperature of the inksubstrate labyrinth to the fix point, with no time allowed for heat transfer before fix takes place. In the case of the cigarette pack, this energy ordinarily sufiicient to destroy the pack, is subsequently dissipated by the cooling spray directly in back of the heated area. This dissipation is nonuniform and may require up to one second to take care of 20 mil thick areas. Since fix has already been accomplished, the only requirement is that the inner surface remain cool and retain structural integrity.

It should be appreciated that in a continuous operating machine, the various elements, such as the shutter and water spray, are synchronized with the positioning of the container in front of the oven face, and means are provided for continuously suplying water to the spray nozzle and removing excess water as the fixing cycle is completed. These techniques for providing automatic and continuous operation are well known in the art, are not considered to be a part of this invention, and therefore will not be described here.

While the specific example of a cigarette container has been used to explain the fixing system described in this invention, it is, of course, evident that this same system with slight modification may be used for many plastic objects both in sheet form, continuous web form, or, as in the present example, in container form. It should also be appreciated that the cooling system need not always be an air-water mist, but in many cases may be a cooled mandrel, roller, fluid-backed membrane, or other such method for removing heat from the nonimage side of the substrate. Furthermore, while a rotating type exposure shutter has been described, the shutter may take on other various well known configurations whereby various desired exposure times and pulse shapes may be obtained. It may also be omitted, the rise time being controlled by motion of a conveyor mandrel past the furnace.

There has accordingly been described and shown herein a novel and useful system for fusing an electroscopic powder image to the surface of a heat sensitive material.

What is claimed is:

1. Apparatus for fusing an electroscopic powder particle image to one surface of a heat sensitive material on which it has been deposited comprising means for applying a predetermined sudden burst of heat energy to the electroscopic powder particle image to fuse the powder, and means for applying a coolant to the surface of said heat sensitive material behind the surface on which said powder image has been deposited while said pre' determined sudden burst of heat energy is being applied to said one surface.

2. Apparatus for fixing an electroscopic powder particle image to the surface of a heat sensitive material as recited in claim 1 wherein said means for applying a sudden burst of heat energy to said powder image includes a source of heat having a radiating area at least on the order of the size of said powder image, said source of heat being positioned opposite said powder image and having a temperature which is higher than that required to melt said heat sensitive material, and movable shutter means positioned between said source of heat and said heat sensitive surface containing said powder image, said movable shutter means having an opening of a predetermined size therethrough, and means for moving said shutter means for permitting said predetermined sudden burst of heat energy to reach said powder image when said opening occurs between said source of heat and said powder image.

3. Apparatus as recited in claim 1 wherein said coolant comprises a mixture of air and water.

4. Apparatus for fixing an electroscopic powder particle image to a surface of a heat sensitive material on which said image has been deposited comprising a substantially black body heat radiator positioned opposite said powder image on said heat sensitive surface, said black body heat radiator radiating at a temperature to said powder image sufficient to cause it to fuse to the surface of said heat sensitive material, shutter means interposed between said black body radiator and said powder image for preventing exposure of said powder image to heat from said black body radiator, means for moving said shutter means in a manner to enable a sudden burst of heat energy to reach said powder image from black body radiator over a predetermined interval to fuse said powder image, and means for cooling all of said heat sensitive surface in back of the region thereof which is exposed to said black body radiator while said powder image is exposed to heat therefrom.

5. Apparatus as recited in claim 4 wherein said heat sensitive surface is the surface of a plastic container having one end open, and said means for cooling comprises means for spraying the back surface of said surface on which said powder image is deposited with a coolant through said open end.

References Cited UNITED STATES PATENTS 1,958,256 4/1934 Yates 118-59 X 2,396,946 3/1946 Grupe 117l19.2 X 2,852,651 9/1958 Crumrine et al. 117-17.5 X 2,882,182 4/1959 Tench 1171 19.4 X 3,053,962 9/1962 Cerasani et al. 117l7.5 X 3,067,586 12/1962 Offen 34-62 X 3,088,386 5/1963 Sugarman 11717.5 X 3,219,326 11/1965 Hunt 219216 X 3,257,226 6/1966 Thwaites 117-119.4 X

WILLIAM D. MARTIN, Primary Examiner.

EDWARD J. CABIC, Assistant Examiner.

US. Cl. X.R. 

